1. Field of Invention
The present invention relates to frequency compensation, and more particularly to frequency compensation in transmitter devices. The present invention is even more particularly related to compensating frequency by multiplying a signal to be transmitted by an exponentially decaying phase shift that matches phase shift due to power source fluctuations.
2. Discussion of Background
When large circuits are switched on or off, the surge in current drawn from the power supply causes the supply voltage to momentarily change. Some circuits are sensitive to power supply fluctuations which can cause undesirable behavior. One such circuit is the phase/frequency locked loop of an analog chip's synthesizer (e.g., a chip containing primarily analog circuits, including a synthesizer). In particular, when the transmit circuits (including the power amplifier) are turned on, a dip in the analog chip's supply voltage causes a slight perturbation in the synthesizer's lock frequency. The magnitude of the frequency error is greatest when the power amplifier is turned on and it exponentially decays to zero with a time constant on the order of 50 μs. The frequency error is imposed on the transmit waveform resulting in degraded performance caused by estimation errors in the receiver.
In the past, three primary methods have been utilized to reduce or eliminate problems caused by noisy circuits interacting with sensitive circuits:
(1) isolate the sensitive circuit's supply from the noisy circuit's supply. Unfortunately, for the case of the power amplifier and the synthesizer, both circuits are on the same die and sufficient isolation is either very difficult or impossible.
(2) time things so that the noisy circuit operates at a different time than the sensitive one. For the synthesizer/power amplifier case, this is a difficult design issue unless the power amplifier is left on for a long time. However, power amplifiers are often shut down to enhance power savings, and specifications often require the time between a transmit command and when the transmitter begins transmission to be a very short period of time, often less than 1 μs. Therefore, leaving the power amplifier on is not practical and other solutions are needed. In addition, specifications can require that the power amplifier not be left on while not transmitting a packet because the transmitted carrier leak would be unacceptable.
(3) compensate for the error elsewhere in the signal path.
Although the third solution is likely the most complex, it is the approach chosen for the synthesizer's exponentially-decaying frequency error during the beginning of a transmit packet because it is doubtful layout techniques could adequately isolate the synthesizer's supply and because leaving the power amplifier on when unused would result in unacceptably-large power dissipation.